Vol. 52.] GEOLOGY OF THE EASTERN CORNER OE ANGLESEY. 



625 



Kar. 9.- 



■Face of cliff, 2J incites below 



the thrust shown at x ,fiy. 7. 



Unless the beds are rapidly folded in boat-shaped isoclines whose 

 axes are inclined nearly parallel to the slope of the surface, they 

 cannot be resting upon their natural base. This, however, is 

 unlikely from internal evidence. The sum of the evidence seems 

 to be in favour of supposing that they lie upon a dislocation- 

 surface inclined to the N.E. at an angle rather steeper than the 

 slope of the ground. This would give the form of outcrop which 

 actually exists. 



Turning now to the rocks seen in the drive, there is abundant 

 evidence that they have been subjected to movement such as might 

 be expected just above such a surface. They are, in fact, in the 

 same broken condition as are those masses of Cambrian and Torri- 

 donian sediments which in the North-western Highlands are brought 

 forward upon one of the major thrust-planes. The accompanying- 

 sections (figs. 7 & 8), on the scale of 50 feet to 1 inch, illustrate this. 

 But they give no idea of the extent to which the fine argillites have 

 been fractured. This is shown in fig. 9, which is taken from a 

 spot 2| inches below the 

 thrust at the point x in 

 fig. 7. Further, along 

 this very thrust-plane 

 is a seam of material up 

 to ^ inch thick, resem- 

 bling in every way the 

 mylonites of the High- 

 land thrust-planes. The 

 same phenomenon is seen 

 in several places. It 

 was difficult to obtain 

 material for slicing with- 

 out defacing the surfaces 

 (I hope that the section 



1 x ' will be respected) ; and slices have therefore been cut from 

 specimens showing similar structure in the Tairfynnon Breccia near 

 Bangor, which it was surprising and interesting to find. These 

 show, under the microscope, perfectly typical mylonitic structures 

 (fig. 10, p. 626). 



The direction of movement is manifestly from right to left in the 

 figures, but as these are not taken along the full dip, but along the 

 crag-faces, it is probably from north-east or east. 



With regard to relative age, the contrast of metamorphic con- 

 dition between these rocks and the schists, on which stress is laid by 

 Prof. Bonney, remains, I think, unaltered ; though perhaps caution 

 may be necessary in view of movements of the nature here described. 

 If, however, as certainly seems most probable, these rocks are later 

 than the adjoining schists, it is obvious that the overthrust sup- 

 posed must be thrusting higher beds upon lower, a reverse of the 



Green banded argillite. Natural size. 



usual effect. This, however, would follow if the 



angle 



of thrust 



were less than the dip of the beds. 



The evidence here given would lead us to expect that a zone of 



